During the formation of a semiconductor device, layers of various materials such as dielectrics, masks, metals, doped polycrystalline silicon, and photoresist are formed over a semiconductor substrate. Contacts through dielectric materials to the semiconductor substrate are typically required during the formation of semiconductor devices, such as memory devices and microprocessors. Contacts to features other than the substrate, such as word lines and other structures, are also commonly performed.
One process to form a contact to a semiconductor substrate or other features through a dielectric includes the step of forming a dielectric layer over the semiconductor substrate, then forming a first patterned layer of photoresist (resist) over the dielectric. An etch defines a contact through the dielectric to the substrate, and the resist layer is then removed.
The contacts are often connected with word lines or bit lines. To form the word or bit lines a second layer of resist is patterned over the dielectric layer leaving the contacts exposed, and also leaving exposed an area parallel and overlying the contacts. The dielectric is again etched, although the dielectric is not etched completely, thereby creating a trench in the dielectric but no additional contacts to the substrate. A layer of conductive material such as metal is then blanket deposited over the surface of the dielectric, and the wafer is planarized to leave conductive material within the contacts and within the trench.
Various problems are associated with processes such as the one described above. One problem is that the dielectric is etched to expose the substrate, then a second patterned resist layer is subsequently formed over the dielectric thus filling the contacts with resist. Due to the small sizes of the contacts in the dielectric, it can be difficult to completely remove the resist from the contacts in the dielectric, and thus the conductive layer which contacts the substrate may not make adequate electrical contact.
One method to solve the problem associated with resist filling the contact has been to first etch a contact in a first dielectric layer, then to deposit and planarize a first metal layer to form a metal plug to the substrate. A second dielectric layer having a trench is patterned over the first metal layer and over the first dielectric layer, then a second metal layer is formed over the second dielectric layer and then planarized. This process, however, requires the formation and planarization of two metal layers, thus adding additional steps and an additional metal-to-metal interface, which can be difficult to form reliably.
A process which can be accomplished by the deposition of a single metal layer and does not require the formation of a resist layer within the contact to the substrate would be desirable.